1. Technical field
The invention relates to the processes and devices for acousto-optic modulation. It is suitable for use in all technical fields where it is necessary to modulate a monochromatic light beam, typically delivered by a laser, an important application being alphanumeric character printing in a laser printer.
Acousto-optic modulators or deflectors, which will be designated hereunder as AOM's, are widely used. In an AOM, a monochromatic light beam is deflected by an angle a which is in direct relation with the frequency f of the acoustic wave generated in the modulator by a transducer responsive to an electrical driving signal. If a plurality of modulation signals at different frequencies f.sub.1, f.sub.2, f.sub.3, . . . are simultaneously applied, the entering monochromatic light beam is divided into an aligned light beam of zero order and a plurality of deflected beams emerging from the AOM along directions at respective angles a.sub.1, a.sub.2, a.sub.3, . . . with the aligned beam. The optical intensity of each deflected beam is a direct function of the power of the respective electrical signal which is applied to the AOM. It is consequently possible to modulate the intensities of the deflected beams by amplitude modulation of the components at different frequencies of a composite electric signal applied to the AOM.
A disadvantage of the current AOMs is that their response is not linear, except for very low efficiencies (efficiency being defined as the ratio between the intensity of the deflected beam and the intensity of the entering beam). For obtaining a significant output power, it becomes necessary to operate out of the linearity zone and consequently:
each time the modulator is used with a high efficiency, there are distorsion and saturation which result in contrast degradation, PA1 if the applied electric signal has a plurality of components at different frequencies, there is cross-talk or cross-modulation.
2. Prior Art
Attempts have already been made for reducing cross-talk in multi-frequency AOM systems by adding a single compensation or dummy electric signal to the input signal having a plurality of modulation channels at different frequencies. That electrical compensation signal has a fixed frequency which is typically selected at a value higher or lower than all frequencies f.sub.1, f.sub.2, . . . of the components of the driving signal. As a result, there is excess compensation for those optical channels which are modulated at frequencies close to the frequency of the compensation signal and is not sufficient for the channels having the frequencies most spaced from the compensation frequency.
That problem still exists and limits the possibilities of the system even when the compensation signal is servo-controlled for maintaining the total electric power applied to the AOM at a substantially constant value, as disclosed for instance in British Pat. No. 2,036,996 (Mori).